1. Field of the Invention
The present invention relates to a magnetometer or a device for detecting or measuring a weak magnetic field (hereinafter, a magnetic field sensor) improved in the effective capture area, in particular a magnetic field sensor having SQUID made of oxide superconductor.
2. Description of the Related Art
SQUID (Superconducting Quantum Interference Device) is a high sensitive magnetic field sensor consisting of a superconducting loop having one or two Josephson junctions. A typical SQUID pattern formed in a thin film of oxide superconductor deposited on a substrate consists of a washer which forms a superconducting current loop surrounding a hole, a pair of Josephson junctions formed in the washer, and a pair of opposite terminals for connecting the washer to an outer circuit. In the SQUID, a variation in magnetic flux of a magnetic field passing through the hole is detected or measured as a variation in output voltage.
In order to enhance the sensitivity or resolution of SQUID type magnetic field sensor, it is necessary to increase the effective capture area (Aeff) of the SQUID through which the magnetic flux passes.
Applied Physics Letters 59(1), July 1991, p123 discloses a SQUID coupled with a flux transformer to increase the sensitivity. This known magnetic flux sensor requires additional manufacturing step to prepare multi-turns of a pickup coil for the flux transformer in a thin film of oxide superconductor, so that the productivity is not high. Another demerit of this sensor resides in that flux transformer made of superconductor must be cooled down to a temperature lower than the critical temperature when it is used.
Japanese Journal of Applied Physics. Vol. 32 (1993), page L662 proposes to use a flux capture plate plate made of superconductor. Use of the flux capture plate plate is, however, limited due to such a fact that it is difficult to prepare in practice a large area thin film of oxide superconductor and hence its size is limited.
An object of the present invention is to solve the problems of known magnetic field sensors and to provide a magnetic field sensor having a simple structure but increased in the effective capture area.
The present invention provides a magnetic field sensor having a SQUID, characterized in that a flux guide made of a material that has a higher permeability than vacuum is arranged in such a manner that a flux passing through said flux guide is coupled with said SQUID.
The flux guide has preferably a height that is three times longer than a diameter of an end surface which is faced closely to the SQUID and has preferably a form of a truncated corn or polygon. The magnetic field guide can be a hollow body or has a through-hole passing from one end surface to another end surface.
The term xe2x80x9ccouplexe2x80x9d includes two cases that the flux guide is coupled directly or indirectly with the SQUID.
In the direct coupling, the flux guide is placed preferably in coaxial with a hole and over a washer of the SQUID. In the indirectly coupling, the flux guide is placed on a pickup coil which is connected to the SQUID, so that the magnetic flux is guided to the pickup coil by the flux guide. Or, the flux guide is placed on a flux transformer which is connected to an input coil with which the SQUID is coupled, so that the magnetic flux is guided to the flux transformer by the flux guide and is sensed by the SQUID through the input coil.
The flux guide has preferably a pair of end surfaces each having a different surface area and a tapered side surface connecting the end surfaces, and one of the end surfaces is arranged in such a manner that the flux is guided to the SQUID and to the pickup coil by the flux guide. A ratio of diameter of an end surface which is opposite to an end surface which is faced closely to the SQUID to the latter end surface is more than 3 times but is less than 15 times.
The magnetic flux guide can be made of Permalloy.
The SQUID, coils, flux transformer and flux capture plate can be made of thin film(s) of oxide superconductor.
An additional flux capture plate can be arranged between the SQUID and the flux guide.
The magnetic field sensor can have a shield plate made of a material having a higher permeability than vacuum. The shield plate is arranged around the flux guide and in parallel with the SQUID.
An essence of the magnetic field sensor according to the present invention resides in that the flux guide made of a material having a higher permeability than vacuum is combined with SQUID directly or indirectly.
The magnetic flux guide used in the magnetic field sensor according to the present invention can be produced very easily because the flux guide has a simple shape, but has improved sensitivity due to its magnetic property or high permeability. Still more, since the flux guide is not made of superconductor, there is no special limitation in its dimensions and any flux guide having desired properties can be obtained.
The magnetic flux guide of the magnetic field sensor according to the present invention has preferably such a height that is three times longer more than a diameter of an end surface which is faced closely to the SQUID, when a shape of the magnetic flux guide is considered as a cylinder.
The magnetic flux guide has preferably such a shape that a ratio of diameter of a larger end surface to a smaller end surface that is faced closely to the SQUID is more than 3 times. By this arrangement, the magnetic flux can be captured much effectively comparing to a SQUID having the same effective capture area. The advantage of enlargement in effective capture area can not be expected when the ratio becomes more than 15 times. Therefore, the ratio is limited preferably within a range between 3 times and 15 times.
The flux guide is preferably made of Permalloy that is a material having high permeability and available on market. xe2x80x9cMaterial having high permeabilityxe2x80x9d means material whose permeability is higher than that of vacuum.
Now, several embodiments of magnetic field sensor according to the present invention will be described with referring to attached drawings. The scope of the invention, however, is not limited by these embodiments.